Uploading an Image via readAsBinaryString() in JavaScript

Question

Is there some offset I should be accounting for when I upload an image in this manner? If i simple send the file, I can write it back out as a valid JPG but if I write the data submitted via readAsBinaryString, the file is approximately 50% larger and corrupt.

Any thoughts?

here's the javascript:

var file = e.dataTransfer.files[0];

var reader = new FileReader();

reader.onloadend = function(event) {
    var xhr = new XMLHttpRequest();
    xhr.open("POST", "/upload/image/"); 
    xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");
    xhr.send(event.target.result);
};

reader.readAsBinaryString(file);

On the server side, I'm using python. If I simply dump the request.raw_post_data I can see the data has been submitted successfully ... problem is that I can't figure out how to write back out to a valid file.

I understand that I'm not handling something appropriately server side, but what?